NEWSYNTHESJS O F
Nov. 20, 1953
[CONTRIBUTION FROM THE
5603
PENTACENE
DEPARTMENT OF CHEMISTRY, WAYNE UNIVERSITY]
Cyclic Dienes. 11. A New Synthesis of Pentacene1i2 BY WILLIAM J. BAILEY3AND MILTONMADOFF RECEIVED JUNE 10, 1953
A general method for the synthesis of linear condensed polynuclear aromatic hydrocarbons is outlined. Pentacene (I) is synthesized in an over-all yield of 30% starting with 1,2-dimethylenecyclohexane (V).
6,13-diketo- A4a(14a),7a(11a) - octadecahydropentacene (VI). All attempts to simultaneously dehydrogenate and deoxygenate V I directly to pentacene failed. Although Cook and Lawrence” converted 9,lO diketooctadecahydro - 1,2,5,6 - dibenzanthracene directly to 1,2,5,6-dibenzanthraceneby heating over platinum a t 300°,similar treatment of V I produced only partially dehydrogenated products that sublimed a t a low temperature. Zinc dust distillation of VI produced a mixture of products containing 6,l&pentacenequinone (X). Since the direct methods failed, procedures for the reduction of the two carbonyl groups were investigated, WoH-Kishner reduction of VI yielded a mixture of products. Chromatography over alumina produced a small amount of the hydrocarbon
Although many excellent syntheses have been developed for angular condensed aromatic hydrocarbons, no good general method has been worked out for the linear condensed ring system^.^ This is due to the fact that most common ring closures tend to proceed in the angular position. Pentacene (I) was first synthesized by Clar and John6 starting from m-xylophenone (11). A vigorous Friedel-Crafts reaction yielded 40% of 4,Gdibenzoyl-l,3-dimethylbenzene (11) from a complex mixture. Heating with copper in a modified Elbs reaction converted I11 into dihydropentacene (IV) in an unstated yield. Dehydrogenation of I V to I was accomplished by boiling the dihydro derivative in nitrobenzene with phenanthraquinone or by passing over copper a t 35O-4OO0.
6
ucH2 0 ‘““() 0
I11
-
IV
I
0
‘CH2
+III+ 0
V
77%
CH2/ Et-S
0
VI
SEt
VI1
ll
0
VI11 Since Cook and Lawrence” were able to synthesize lJ2,5,6-dibenzanthracene from l-vinylcyclohexene and quinone, it was considered likely that a synthesis of pentacene could be worked out using the readily available 1,2-dimethylenecyclohexane (V)2 and quinone. Thus it was found that refluxing V with benzoquinone produced a 7’7% yield of (1) This work was supported in part by the Office of Naval Research. Presented at the 119th Meeting of the American Chemical Society, Boston, Mass., April, 1951. (2) Previous paper in this series, T H I a JOURNAL, 71, 4780 (1953). (3) Department of Chemistry, University of Maryland, College Park, Md. (4) E. Clar, “Aromatische Kohlenwasserstoffe,” Springet Verlag, Berlin, 1941. (5) E. Clar and F. John, Be?.. 68, 3027 (1929); 68, 2987 (1930); 84,2194 (1931). ( 0 ) J. W. 60tlk atid 6 , A , Lawrenre, J , Ckenc, ~ o c , 68 , fleas),
IX
XI
VI11 plus a yellow solid. Probably the base rearranged the double bonds to produce the quinone XI which would not be expected to produce the hydrocarbon under the Wolff-Kishner conditions? An excellent method for removing the oxygen atom was developed by applying the method of Wolfrorn and Karabinoss for the desulfurization of the corresponding thioketal. Treatment of V I with an excess of ethyl mercaptan in the presence of zinc chloride produced the crude tetraethyltetrathiodiketal (VlI), which was not purified but treated directly with Raney nickel to produce VI11 (7) D. Todd in R. Adams, “Organic Reactions,” Vol. IV, John Wiley and Sons,Inc., New York, 1948, p. 378; J. Thiele and P. Barlow, Ann., 808, 311 (1898). (8) hl, f, Weltrorn and 1, V , Xerabttlor, Tmo JOURNAL, OI, 009 (IVdd),
5604
WILLIAM J. BAILEYAND MILTONMADOFF
Vol. 75
B. By Desulfurization of the Dithioketal.-Following the desulfurization procedure of Newman and Walborsky,lo 1.7 g. of anhydrous zinc chloride was placed in 50-ml. flask and fused under vacuum. After adding 3 g. (0.011 mole) of the above diketone VI, 1 g. of anhydrous sodium sulfate and 35 ml. of ethyl mercaptan, the flask was well stoppered and shaken a t room temperature until the zinc chloride dissolved. After standing overnight at room temperature, the reaction mixture was heated to remove the excess ethyl rnercaptan. The residue was taken up in ice-water and ether. The aqueous layer and insoluble material (zinc oxychloride) were extracted thoroughly with ether. The combined ether extracts were washed with 10% aqueous sodium hydroxide solution, water and saturated sodium chloride solution. After the solution was dried over anhydrous sodium sulfate, the ether was removed by distillation leaving the crude dithioketal. This crude dithioketal was dissolved in ethanol and added to a susDension of 50 IC. of Raney nickel catalyst (W-4)lI in a toti1 volume of 175 ml. of ethanol. After ihe mixture had been refluxed for about 12 hours, the catalyst was removed by centrifuging and thoroughly washed with hot benzene. The benzene washings and alcoholic filtrate were Experimental evaporated to dryness and the resultant white solid recrys6,13-Diket0-A~~(~@)J~(~~~)-octadecahydroqentacene (VI). tallized from a benzenealcohol mixture to yield 1.4 g. (55%) -In a 50-ml. round-bottom flask was placed a solution of of A b(la)Ja(lh)-octadecahydropentacene (VIII), m .p. 131132'. 5 g. (0.048 mole) of 1.2-dimethylenecyclohexanez and 2.5 g. (0.024mde) of benzoquinone in 25 d.of dioxane (puriAnal. Calcd. for C ~ ~ H I C, P : 89.19; H, 10.81. Found: fied). The solution was heated under reflux until a precipi- C, 89.49; H, 10.74. tate formed (2 hours) and then an additional hour. The Pentacene (I).-A mixture of 75 mg. (0.00026 mole) of precipitated adduct was filtered, washed well with ether and octadecahydropentacene (VIII) and 100 mg. of 5% pallarecrystallized from chloroform-petroleum ether (30-60'). diurn-on-charcoal was heated at 245' in a test-tube with two There was obtained 5.8 g. (77%) of fluffy white needles of side arms and a cold finger until the calculated amount of 6,13-diketotictadecahydropentacene(VI), m.p. 225-226'. hydrogen (96%) swept out with carbon dioxide was colAnal. Calcd. for CptHsa02: C, 81.48; H, 8.64. Found: lected in a gas buret containing 30$& potassium hydroxide solution. Some material sublimed during the dehydrogenaC, 81.39; H, 8.78. A@(1&)Ja(lla)-Octadecahydropentacene (VIII). A. By tion. This was recovered and converted to pentacene by Wolff-Kishner Reduction.-A mixture of 2 g. (0.006 use of a larger quantity of catalyst. The combined mixture mole) of the above diketooctadecahydropentacene (VI), of catalyst and pentacene was transferred to a 100-ml. three1.84 ml. of hydrazine hydrate (80% aqueous solution), 10 neck round-bottom %sk fitted with an inlet capillary tube, ml. of triethylene glycol and 1.2 g. of potassium hydroxide a cold finger, and an exit to the vacuum pump. The pentawas heated under reflux for 1.5 hours according to the direc- cene was sublimed onto the cold linger in a stream of carbon tions of Huang-Minlon.9 The water was then drained from dioxide a t 5-8 mm. pressure by heating the flask in a Woods metal-bath a t 270-300'. The yield of the violet-blue pentathe condenser, the temperature was allowed to rise to 195', and the refluxing was continued for 4 hours. The cooled cene (I) was 52 mg. (72%). In accordance with the obsersolution was diluted with 20 ml. of water and poured slowly vation of Clar and John6 the pentacene (I) sublimed below into 10 ml. of 6 N hydrochloric acid. The resulting yellow its melting point. Anal. Calcd. for C22Hll: C, 94.96; H, 5.04. Found: solid was filtered and dried (wt. 1.80 g.). On recrystallizing from a benzenealcohol mixture, the5e C, 94.68; H, 4.82. was obtained a light yellow-orange solid, m.p. 110-143 , Maleic Anhydride Adduct of Pentacene (IX).-A mixture which could not be purified further by recrystallization. of palladium-on-charcoal catalyst and pentacene prior to The colored material from several runs (2 g.) was com- purification [obtained from 55 mg. (0.0002 mole) of octabined, dissolved in benzene, and chromatographed on a 30 decahydropentacene] was suspended in 6 ml. of xylene acX 2 cm. alumina column. The column was eluted with 50- cording to the directions of Clar.6 To this suspension was ml. portions of benzene until the yellow band had moved added 32 mg. (0.00032 mole) of maleic anhydride, and the down the column and begun to appear in the eluate. These solution was heated to boiling for 10 minutes. The hot eluates (which had a slight violet-blue fluorescence) were solution was filtered and the adduct crystallized on cooling combined, evaporated to dryness, and extracted with 100 to yield 63 mg. (92%) of the microcrystalline needles of IX. ml. of petroleum ether. The extract was then evaporated This adduct was recrystallized from a chloroform-petroleum to dryness and the residue was recrystallized from chloro- ether mixture to yield white crystals, m.p. 295', decomposform-petroleum ether. There was obtained 34 mg. of white ing to a red liauid in agreement with Clar's observations.6 needles, m.p. 127-128'. Mixed melting point with an auCalcd. for C2~HlsOl: C, 82.98; H, 4.26. Found: thentic sample of Ac(*@)~7a(11a)-wtadecahydropentacene C ,Anal. 82.78; H, 4.01. prepared below showed no depression. The yellow subCOLLEGE PARK, MARYLAND stance formed during the reaction was assumed to be the quinone XI but was not characterized.
in an over-all yield of 55%. The hydrocarbon VI11 was smoothly dehydrogenated to pentacene (I) by heating with 5% palladium-on-carbon for 3 hours a t 245'. Purification of I by sublimation a t 300' (5 mm.) in a stream of carbon dioxide afforded pure pentacene in a 72% yield. The violet-blue pentacene was characterized by analysis and conversion to its maleic anhydride adduct IX. The properties of these two products agreed with those previously reported by Clar and John.6 Thus, this direct, easy synthesis of I in an over-all yield of 30% from 1,2-dimethylenecyclohexane(V) appears to be a considerable improvement over previous methods. Since the procedure used in the synthesis of the cyclic diene V is general, this method can be applied to other linear condensed polynuclear aromatic hydrocarbons.
(9) Huang-Minlon, THISJOURNAI, 68, 2487 (1846).
(10) M. S. Newman and H. M. Walborsky, ibid., I!&4296 (1950). (11) A. A. Pavlic and H. Adkins, ibid., 68, 1471 (1946).
